Facebook might be one of the most massive social networks in existence, but it's a site that isn't known for its promotions or contests--because they don't actually hold any of these. Rather, any competitions or raffle programs you see being advertised on the social network are the handiwork of some very creative cyber criminals.
This includes the Facebook 'Instant Rewards' scam, as reported by Hoax Slayer, which claims to award winners with up to $50,000 in cash.
Scammers will select random users and send them messages, informing them that they've won fifty grand with a 'Facebook User Notice' heading:
CONGRATULATIONS! Your $50,000.00 reward on Facebook. Please CONFIRM YOUR EMAIL ADDRESS HERE: [link]
Other prizes include gift cards and non-existent cash prizes in lower denominations. The provided link takes the users to a page which asks them for their email address. Then they're redirected to another page with a longer form which makes no mention of the prize anymore.
One of the fields asks for the user's phone number, and this is where the user will eventually get screwed over. When the number is sent over, the user is automatically subscribed to a number of pay SMS subscriptions, which will cost a ton when added up.
Scams such as these are on the rise, so when in doubt of any contest or offer, do a quick search on Google and see what turns up. Most likely, what you've got there is a scam.